text:
Which aerobatic maneuvers are most likely to make people airsick?
I'm interested in classifying aerobatic maneuvers by their propensity to cause airsickness. While there will be some variation across a population, I think we can use first principle arguments to show that low-gee loops are much less nausea-, dizziness-, and vertigo-inducing than snap rolls. Likewise, a single 1G barrel roll is much less disorienting than extended inverted flight.
I'm hoping that in a similar vein to how we have documented human physiology during high-acceleration events, there will also be some literature on physiology in aerobatic environments.
(For the purposes of this question, I'd like to put anecdotal experiences aside, or at least limit them to the comment section. Certainly there are outliers who have little trouble with the hardest maneuvers but a lot of trouble with the easiest.)

Motion sickness usually results from conflicting sensory information. Your inner ear doesn't match what your eyes are seeing. I would guess barrel rolls would be the absolute worst. Your eyes see you spinning around yet your vestibular sense feels that you are upright the whole time.

Amongst other things, it matters a lot whether the person is in control (i.e. the pilot or the passenger).

Generally, in my (somewhat professional) experience, your "first principle arguments" are not true. For many people, light and particularly repeating accelerations are more nauseating than strong loads per se. The disconnect between the vestibular and visual information that @TomMcW mentioned is more important. But anecdotally, spin must be one of the worst.

Any chain of aerobatics is increasingly likely to induce airsickness as time progresses. I don't think that refutes the first-principles claimed above, since extended periods of snap rolls or spins are going to have an even stronger disconnect between vestibular and visual. I'm hoping to find data which goes beyond "many people" to include "people in general".

Supporting TomMcW, "Sensory conflict theory holds that, when [optical] perception of self-motion is not corroborated by inertial forces transmitted through the vestibular system, simulator sickness is likely."

My experience is that passengers lose their cookies when you do a steep, turning descent

The most widely accepted explanation for motion sickness (which includes Airsickness, seasickness, carsickness, simulatorsickness and since recently selfdriving car sickness) is the sensory conflict theory. This theory states that if you sense a different motion via your vestibular system (the inner ear), then what you see via your visual system (your eyes) which does not align with your experience, then you will encounter motion sickness. It is important to notice that this mismatch is matched with your personal experiences of motion, therefore you can get acustomed to certain types of motion after having experienced them a couple of time. In other words: You learn to accept a given motion, which then will not make you motion sick anymore (or in some cases, not as much). Additionaly, people are especially suceptible to certain frequencies of motion. The Standard ISO-2631 (unfortunately restricted access) states that 0.2 Hz oscillations are especially nauseating.
Given this background, you will have to identify a maneuver which results in the biggest motion conflict possible and ideally features a lot of motion in the 0.2 Hz range. Because there are currently only very unsufficient models of this motion conflict (which is an ongoing research topic, one model is described here for example) it is imposible to classify which flight maneuvers are best suited for this task (or at least with good accuracy). I would propose motions with unexpected centrifugal forces, and unexpected orientation turns. Centrifugal forces screw up the correct sensing of Earth's G-Vector by the vestibular system which then is interpreted (by a lot of people) as a unexpected motion. Because of a lack of accurate models it is not possible to foresee which maneuver is the most nauseating, but I would propose a stall turn (or hammerhead) as it ticks all these boxes.
However when going back to the ISO-2631 I can speak from personal experience that the absolute fastest way to make people airsick is by flying a vertical standing sinusoid flight-path with a frequency of 0.2 Hz. This is an incredibly fast way of making people airsick.
One note given your sentence:

Likewise, a 1G barrel roll is much less disorienting than extended inverted flight.

Motion sickness research suggests that this is not true, simply because the human motion sickness mechanism is centered around oscillation in some sort. Extended inverted flight is a static condition while a 1G barrel roll is a motion with a distinct frequency spectrum, which would be more adapt in provoquing motion sickness...

Positive to zero gee cycling, even far from a 5 second period, will do it reliably to almost everyone, even with experience, if continued long enough.  They don't call it the "Vomit Comet" for no reason...

@U_flow, great answer. I updated my question to clarify that I was referring to a single perfectly-coordinated barrel roll-- much like an amusement park ride-- , as opposed to the discomfort, disorientation, and discombobulation stemming from a long (minutes) period of inverted flight.

This is consistent with my experience. Specifically, it is the downward leg of the flight path where the vomiting occurs.

Would you please describe a vertical standing sinusoidal flight-path?  Would that be the path taken by an aircraft pitching up and down in a sinusoidal fashion?  If so, what does "standing" mean?

@rclocher3 the flight path I describe is an aircraft pitching up and down in a sinusoidal fashion. With standing I meant to describe "vertical" therefore in an up-and-down fashion, but "pitching motion" describes it more clearly.

The answer by @U_flow is very good.  But, I want to add a little to it.  The NASA KC-135 was referred to as the "vomit comet" for a good reason.  NASA flight surgeons would insist that everyone keep an air sickness bag visibly sticking out of an easy access pocket.  They also offered everyone prescription motion sickness medication (Scope-Dex).  I know of at least one experienced pilot and astronaut who proceeded to fill up a bag while flying on the "K-bird".  That aircraft was flying parabolas, but there is little outside visibility from inside most of the aircraft.
Meanwhile my experience of flying inverted has been less disorienting than other aerobatics.
I have also seen motion sickness, as well as spatial disorientation (which are related), when in instrument flying conditions.  Flying in the clouds causes a loss of visual reference and can lead to a sensory conflict.
As you might expect experience matters.  People can get used to different things.  Part of that is probably due to the expectation of what will occur.  You can learn to expect "which way is up" and the correct direction to glance to find the horizon.  For an experienced pilot, who is not flying, just keeping a light tough on the controls provides additional feedback about what the other pilot, and therefore what the aircraft, is actually doing.  That can be very helpful.
The worst one is probably one that you have never done before, with some light turbulence added, without a clear visible horizon.


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Restore air traffic controllers' instructions from aircraft tracks
Is it possible to understand what commands had been given to an aircraft if you look at its track containing information about lat, lon, altitude, speed and course? Is there anyone who has already tried to do this thing?
What do you think can be a problem? I suppose it may be ambiguous sometimes. For example, heading command and proceed to a fix command can both result in changing heading.

No - there are just simply far too many permeation's of instructions.
For example - an aircraft climbs 1000ft. This could be because:

It has just received an instruction to do so
It previously received an instruction to do so at a certain point
It is operating within a block clearance and chose to do so
It experienced an emergency or TCAS resolution instruction and did so without instruction
It made a mistake/pilot deviation and incorrectly did so

And that's just one minor change, with one aircraft. You can certainly take a guess on some of them but even ignoring unusual situations the first three possibilities are common and normal practice.
And that's just traffic presumed to be operating IFR within controlled airspace - add in VFR traffic in uncontrolled airspace (And bear in mind that they can be still be receiving instructions and clearances at that point - for example, entry into controlled airspace or landing at an airport in uncontrolled airspace) and you've got, essentially, no chance.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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Are there any publically available recordings of conversations between ATC and pilots?
It can be either audio or transcription.
It would be perfect if it also contains aircraft tracks.

which country are you asking about?

You can start with Youtube: https://www.youtube.com/results?search_query=atc+audio

Are you interested in recordings only or live streaming as well?

Location doesn't matter. Preferably recorded not live data. Transcribed recordings are the best thing for me. However, most of the transcriptions that I've found are about some incidents but I need something typical.

What are you trying to do with the recordings/transcriptions? Knowing that might help determine a source, or perhaps alternative way to accomplish your goal.

LiveATC.net has live and recorded ATC radio for most countries/airports.  There are lots of flight tracking websites, e.g. FlightAware, that have live and recorded radar/ADS-B tracks.  You'll have to correlate the two yourself.

I'm a PPL student and I use two sources:

LiveATC.net web site for getting used to live communication on my local airport and also for analyzing recorded communication during my training flights.
VASAviation YouTube channel, which contains many videos capturing communication during aviation incidents, including charts and subtitles.

Yes: Many publicly available accident/incident reports contain communication transcripts; some of those will contain aircraft tracks, too.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Would large, low aspect ratio fan/propeller blades be more efficient? Why not?
With reference to this answer
We read in the above that accelerating a larger mass of air by a smaller amount is (theoretically) more efficient. This is an advantage of a larger propeller than is current practice.
However, we also read above that at some point in increasing size, parasitic drag begins to outweigh the benefits of a larger airmass.
So, what if we used propeller/fan blades with larger surface area, but did not maintain the very high aspect ratios we see today? This would increase Reynolds number, decreasing skin friction drag coefficient. In fact, large-chord fan blades are already being rolled out, though more for simplicity than the above reason.

I'm not sure I entirely understand what you are asking.  Maybe you could draw a picture of current design vs what you have in mind?  Also props vs ducted fans are similar, but different. Maybe narrow down to one or the other to make the question more clear.  Also not sure about "In fact, large-chord fan blades are already being rolled out," they have been around since at least the mid 1990s, and they are anything but simple.  A single blade for a GE9X costs on the order of $100k to manufacture.

you might like to read about 'disc area' referring to features of propeller theory.

Related: https://aviation.stackexchange.com/a/15383/4108

Low aspect ratio means more chord. More chord means

lower lift coefficient. This will move the operating point of the blade airfoil to a less efficient polar point. Since torque and RPM stay constant, a wider blade cannot absorb more energy and compensates for the higher chord with a proportionally lower lift coefficient.
more area. You rightly note that the higher blade Reynolds number reduces the friction drag coefficient, but drag is coefficient times area (times dynamic pressure), so drag will still rise compared to a more narrow blade.

Higher chords are sensible for high-speed propellers which are designed at operation close to Mach 1, where a high lift coefficient will cause higher overspeed and earlier shocks. The most notable example is the Aerosila SV-27 (СВ-27 in Cyrillic) contra-rotating propeller of the D-27 propfan engine powering the Antonov An-70:

SV-27 propellers on the An-70 (picture source). Eight blades in the forward disc and six in the rear, running at only 1200 RPM. In order to reduce Mach effects, all blades have a swept tip and deep chord. Another example is the Junkers VS-9 propeller from 1944.

Junkers VS-9 propeller (picture source). Since this was powered by the same Jumo 213 engine which powered the FW-190D, the deeper chord meant a proportionally lower lift coefficient when compared to the regular FW-190 propeller.

Hi Peter, Aerosila and Cthomas links don't seem to be working. Some SV-27 info is here http://aerosila.ru/en/products/vozdushnye-vinty-ivintoventilyatory-dlya-samoletov/vintoventilyatory-klass-tyagi-10000-kgs.

@ElectricPilot Thank you - I didn't check the link when I took it from an older answer. Interestingly, the prop now is called CB-27. Now it links to the site you found.

The original Russian title is "СВ-27", but Cyrillic "C" and "B" are normally translated into English as "S" and "V", so I think using "SV-27" would probably be clearer in a English-speaking version of their site.

@ElectricPilot Thank you, I should had figured out that myself but missed it.

In the case of propellers, there are a number of operating and design considerations which must be taken into account here.
For example, while it is true that imparting a small impulse to a large mass wastes less energy than adding a large impulse to a small mass using a propeller, there are practical limits to how big you can make the propeller diameter before the ground handling of the airplane becomes unmanageably awkward. A huge prop might be more efficient, but you'd need a boarding ladder to get into the cockpit behind that huge prop and the landing gear struts would need to be longer and stiffer and hence heavier. Oh yes- and the prop itself would be significantly heavier.
In addition, to drive a big prop slowly and still develop full power with a piston engine requires a reduction gear of some sort between the 2400RPM of the crankshaft and whatever the turning speed of the slow prop was. The gearbox adds weight and requires an overhaul when its time is run out, and that adds cost- and a potential failure mode that is absent in direct-drive designs.
Finally, gearing down the engine to turn a slow prop magnifies the transmitted torque to the prop and therefore from the prop to the engine and airframe. That reaction torque has to be countered somehow or else opening the throttle will not just turn the prop but also roll the aircraft backwards around the crankshaft axis.
100 years of experience with geared and direct drive piston engines and all sorts of props has yielded the best solution in the form of a direct-drive engine spinning a variable pitch prop with a diameter that puts the blade tips comfortably subsonic at the max power setting of the engine.

More modern piston engines are often geared anyway (e.g. the Rotax 912), because running at higher RPM gives higher power-to-weight ratio and of course all turbines are (PT6 power turbine runs around 30,000 RPM). (Fixed) reduction gearbox isn't really a big problem.


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Why does the mass penalty of cantilever vs externally braced wing decrease with aircraft size?
From Wikipedia
Cantilever wings require a much heavier spar than would otherwise be needed in cable-stayed designs. However, as the size of an aircraft increases, the additional weight penalty decreases.
Why? The article does not have a citation there.

Speculating that the increase in spar size to cantilever a large wing vs having it cable-stayed is much smaller than the increase in spar size to cantilever a small wing vs cable-stayed? i.e. the difference in size and weight of the spar isn't that much when the wing is large, especially when offset by the lack of drag and weight of the stays.

My guess is that Wikipedia is wrong here, I'll add a "citation needed" tag to that claim. Recall that the Gossamer Condor and Gossamer Albatross man-powered monoplanes had large spans and still had to be wire-braced.

While adding a citation tag I noticed that the whole section of the article has been tagged as unreliable and, reading it through, it is full of little mistakes and muddles. I'll try and give it a proper makeover if I can find the time, meanwhile I'll give a provisional answer here.

I think the Wikipedia article is wrong. Scaling the structure has other issues, but spar weight is not one of them.
The correct approach is to compare the scaling-up of a cantilevered monoplane wing with the same scaling-up of the equivalent wire-braced monoplane design.
In both cases, structural weight rises with the square of the linear scaling, for example doubling the length and span will give a fourfold increase in structural weight. The designer's challenge is to maintain the same weight of structure per unit of wing area, called wing loading, at scale.
The real difference comes not with scale but with speed. As the speed rises, the drag of the bracing rises more sharply, while the aerodynamic forces on the wing demand a stronger structure, especially thicker skinning. Eventually you reach a point, at around 200 mph (300 kph), where the drag of the bracing becomes excessive and the strength of the airframe will admit a cantilever construction without excessive additional weight penalty.
This was what triggered the historic change in design habits which the Wikipedia author was trying to explain and is, I suspect, what they would have said had they known better.
I have now substantially reworked that section of the Wikipedia article (current version here) and added a couple of citations.

so higher speed=thicker skin=stronger wing. right?

Well, it's half-right. Things like the resistance of the skinning to local knocks, scuffs and wrinkles can also be important. These matter more when the skin is structural. If your wing skinning can take that kind of treatment at 200 mph then you are more than half way to a stressed-skin cantilever: it's worth going the rest of the way.

@Guy Inchbald appreciate your excellent work.  Please check "maintain...wing loading at scale".  This is not possible unless it is accompanied by an increase in speed.  Hence, higher G loads when maneuvering lead to the need for a stronger spar.  Late 1930s aircraft were literally ripping their wings off in high G maneuvers, sadly, in some cases, this lead to a trend toward smaller tails.  But the spars did get stronger.

@RobertDiGiovanni. Sorry, that appears totally confused. It is well known that the square law in scaling apples at constant airspeed.

Something about area squares and weight cubes.  Looking at wing loading trends for larger aircraft (that also go faster).  Notable exception is the horse, which is as graceful as a ballerina on its toe(s) because of its suspensary ligaments, which support, absorb shock, and provide spring.

That's the whole point. Volume cubes, but with a little ingenuity the weight only squares. It's why there is more space per passenger in a jumbo jet than a feederliner. You'd need to ask a new question to get a fuller answer.

I would go back to wing loading.  It is very rare to get lower wing loading on a larger aircraft.  More fuel, bigger engines, greater payloads, and higher speeds.  I believe you are correct with empty weight, MTOW may be another story, but + answer anyways.

You appear to misunderstand what wing loading is: it is the weight supported by a unit area of wing. Or, if you like, wing loading = aircraft weight divided by overall wing area. You'd need to ask a new question to get a fuller answer.

The speed influence on the structure itself is neglectable since the stresses due to chordwise loads (drag, thrust) are some orders of magnitude smaller than the ones due to the lift. Take a look at wing mass estimation formulas (e.g. Torenbeek, Nicolai): neither one of them has the aircraft's speed as a parameter. I agree that speed is the reason to drop strutted wings, but due to the need to carry extra fuel and not because a thicker structure is needed.

@Gypaets Yes, of course l/d is the important thing, not the speed at which it is obtained. The weight growth is more to do with the stiffness and hence thickness of the skin than of the internal structure, and as Robert DiGiovanni points out the higher speeds also tended to create a requirement for higher G-force specifications. Perhaps I am wrong, but I would expect these to be used directly for estimating, rather than plugging in the speed which led to the need for them.

I "appear not to understand wing loading", Guy, that's digging a bit low.  I will show you wing loading charts of larger planes and see if you appear to understand them.  But that is rather simplified.  With larger planes you get longer lever arms too.  Pissed me off the way Helios was lost.  Do you think a cable might have helped that?  And BTW volume is useless unless it is carrying fuel or payload, which weigh something, so...

@GuyInchbald Higher speeds don't correlate with higher load factors: the load factors for certification of large, fast flying airplanes (usually about 2.5g) are smaller than the ones for light and slower aircraft (usually 3.8g, for CS-22 even 5.3g)

@Gypaets You muddle speed with size. Size just happens to correlate with speed in the modern civil market (though it tends to go the other way in the military). Historically, maximum load factors did increase fairly steadily with aircraft speeds until the late 1930s, which is the era Robert's remark referred to.

Interesting, because the largest bridges are cable stayed.  Any design depends on anticipated G loading and top speed.  Not sure if this "engineering by rote" maxim belongs in anyone's concept toolbox.
Stayed cable designs suffer from increased drag with increasing speed, but the major penalty is the inefficiency of 2 wings compared to one, and the further penalty of having to carry a heavier, more powerful engine for similar cruise performance, with another penalty of having to carry more fuel to get the same range.
Larger planes are generally not designed for fighter plane-like G loading, and a slightly heavier internal spar on a more efficient single wing become advantageous.

Hey, thanks for the answe. Boeing SUGAR has a strut-braced wing. With jury struts too.

The difference in g-loading is due to the lower roll/pitch/turn rate?

@Abdullah yes, sort of like driving the family car instead of racing around.  These planes can be built to save weight and drag.  Higher aspect wings too.


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Does flutter disappear above the flutter speed?
Okay, lets assume we have an airplane with some sort of superstrong wing, but otherwise conventional configuration. We accelerate it gradually. Eventually, it will reach flutter speed, at which the wingtips will flap up and down with ever increasing amplitude in a standing wave. But we don't stop there - we keep accelerating. 1.5x flutter speed. 2x flutter speed. 5x flutter speed. 10. What happens to the flutter? Does it disappear, then reappear again with more peaks and troughs of the "standing wave", at every multiple of the initial flutter speed? Or does it continue to get worse and worse?

Flutter happens when elastic and aerodynamic frequencies converge. Normally, the elastic one is (almost) fixed and the aerodynamic one increases with flight speed. When speed increases further and the frequencies move apart again, flutter will die down. However, with further increasing speed more elastic modes will be encountered with ever smaller gaps between them.
There are indeed cases of airplanes which have a flutter mode below minimum speed but are safe to operate within their certified speed limits. The linked article doesn't mention this, but if you fly that thing slowly enough, the tail surfaces will wildly swing caused by a torsional elastic mode of the rear fuselage being in frequency neighborhood with oscillating separations on the tail surfaces. Pretty scary but harmless at that low speed.

The Breezy I flew back in the 70s would do that because the skinny truss wan't very torsionally stiff, and you avoided looking back because the oscillations of the J-3 Cub tail assembly it had, at certain power settings and speeds, would scare the bejeebies out of you.  It seemed to be mostly the natural frequency of the truss matching the beat frequency of the pusher prop's pressure waves, only about 5 feet ahead.

ever smaller gaps means smaller speed increments, right?

@Abdullah Yes, right.

@JohnK And now translate this to a GFRP fuselage which is less stiff and tolerates several percent of elastic elongation.

I've witnessed that in a Libelle that flew overhead at my field abt 500 ft at Vne, dumping water at the end of the day, and there was a BRRRRR sound that lasted for about 1 sec, sort of like a drum roll sound, maybe 20 hZ.  The pilot landed safely, with no apparent damage to the glider, somewhat alarmed to say the least.  The Libelle's almond shaped cross section, with the narrow axis the lateral one, seems to be particularly prone to that sort of thing.

@JohnK: Scary indeed. The Libelle is rather lightweight and delicate, so even when this lasted only a second I would be concerned about the control linkages and their attachment points. After the flutter movie with the SB-9 was finished, the plane was inspected and the aileron linkages were found to be almost worn out. The next flight would had most likely resulted in a torn lug in the aileron linkage.


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How much might soft skin reduce aircraft skin friction?
According to Peter Kampf's answer here, soft skin can damp a (vibrational?) mode that cause laminar flow to become turbulent. Thus, it could help delay boundary layer transition. My question is, for a modern large transonic jet (737+), how much may this reduce drag?

This has mostly been examined for aquatic flow, e.g. with the soft skin of dolphins. Studies were done with test setups because measuring the boundary layer around live dolphins is exceedingly hard.
One study compared a rigid cylinder with a soft one of the same dimensions, using a polyurethane coating. Below is a diagram from that study with 3 denoting the rigid and 6 the soft cylinder. As you can see, the difference is small but consistent over the range of Reynolds numbers tested. Picture source here.

For dolphins, active regulation of muscle tension might be responsible for a resonant vibration of the skin, but much of the literature is still rather speculative. From the study:

Based on experimental research on live dolphins, a hypothesis has been formulated (see Chapter 1); this is that by the regulation of the skin muscle tension dolphins promote resonant interaction with a flow. This leads to interaction of fluctuations of the boundary layer and natural frequency of fluctuation of the dolphin skin. As a result, the skin coverings vibrate with resonant frequency practically without energy consumption. Fluctuations of this coating occur at a frequency that controls the frequency of bursting from the viscous sublayer. This is also the reason for the abnormally low resistance, which has been discovered in the authors’ modeling experiments.

This is also the reason for the abnormally low resistance, which has been discovered in the authors’ modeling experiments. What the heck, so dolphins massage the flow around them to reduce drag?

@Abdullah: Sounds like it. But resonance means that almost no work is done and I think this finding is still rather speculative.

This may be where Reynolds number can get tricky.  Take a look at the face of someone with exposed skin in a rocket sled.  Flutter?  Anything soft has the potential to deform, store energy, recoil, etc.  But birds may have some success with shaping their body form in the airstream with feathers.  But at higher "velocity dominated Reynolds numbers" (just made that up) soft and pliable may not be the way to go.


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What are the advantages/disadvantages of a forward-folding propeller?
The AMS Carat A is a motorglider with a forward folding propeller.

To my knowledge, this is the only production plane with this characteristic. The most popular self-launching gliders with front motors use props which fold back.

A naive guess is that aerodynamics come hugely into play here, although I don't have a notion if they're better or worse than a rearward folding prop.
What are the advantages to this folding style, beyond the fact that a rearward folding prop would simply not be practical on an engine-cowled nose?

I think this is a lot more practical than it is anything else.

I don't think aerodynamics would make so big difference. The forward folding propeller needs a good lock, but can stop at any angle, while the aft folding propeller should stop at suitable angle (but maybe its short enough that it does not matter too much), but is held in place by aerodynamic forces in both positions.

I would like to know if it always stops in the same orientation (flat faced aimed mostly at ground-sky in picture).  If sometimes stops 90 degrees to that, seems like aerodynamic forces might be significantly different.  Maybe...

The problem has traditionally been that if you wanted to minimize propeller drag on a cowled tractor motorglider engine you had to go with a feathering prop.  Expensive, and a bit heavy because you have to have a hub with blades that can rotate on their axes to change pitch, with a passive or active pitch change system, so the hub needs a blade root retention structure with bearings that allow the blade to rotate on its axis and a control mechanism of some kind.
With an aft folding prop it's a simple job to hinge each blade on a bolt to passively trail, and when it's spinning the blades naturally move out and rest against structural stops or just sit in the proper position from centrifugal force.  You don't even need an actuating system because the wind blows them back for you when you're done.
This design tries to achieve a similar benefit but needs an active method to drive the blades forward against the air flow, so it adds an internal gas spring mechanism in the hub to push the blades into the forward position against the air flow when not spinning, and when spinning centrifugal force is enough to keep the blades from coning too much from thrust loads plus the gas spring (the VW based engine spins pretty fast, 3000 rpm, so there is plenty of centrifugal force).
So the main advantage is you get most or all of the benefit of a feathering prop (the only other option), without the elaborate hub/bearings required for feathering, just a simple pivoting bolt or pin and a spring system to push the blades forward.  The spring system is the only add-on relative to a super simple aft-folding prop.  Pretty elegant.
As a main disadvantage, it's easy to imagine that the blades when folded will be a little bit destabilizing either in yaw or pitch or both by adding surface area forward, depending on where the flat surface is oriented when it stops (like the way floats are mildly destabilizing in yaw and usually require extra fin area).  No doubt that is taken into account when sizing the tail surfaces and perhaps the blade surface area is too small to be noticeable.
The other problem is the engine can't be made to windmill, but that's also a problem with an aft folding prop as well. and in any case the VW based engine this airplane uses won't windmill even with a fixed prop (if the engine quits in your VW powered homebuilt, it'll stop at just about any speed you can fly at and you need a starter to get it going again).


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Does having permission waive the 500 foot distance requirement in 91.119(c)?
The FAA expressly forbids flying within 500' of people, structures, etc...:

§ 91.119 Minimum safe altitudes: General.
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
.
.
.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

It's hard to find much wiggle room in the plain meaning of this statement. However I feel like there would be an exception when either the structures in question are your own (or otherwise authorized by the owner) or the people on the ground are somehow participating in the flight (such as judges for a flour drop).
Nonetheless, no exception exists in 91.119(c). Is there an exception anywhere else, perhaps in a LOI or AC?

Owning a structure doesn’t make you less likely to hit it. If you can show a good reason to get closer and adequate clear space nearby to land in the event of engine failure, you should be able to get a waiver under 91.905.

IMHO, the FAA doesn't particularly care if you hit things. It cares if you hit things and as a result other people care. Many of the FAA regulations are built around ensuring that public outcry does not bring about a congressional act which constrains the FAA to act in a certain manner. For instance, practice as much aerobatics as you want, so long as you don't scare the public or hurt anyone aside from yourself. (This opinion is motivated in largest part by a direct 2015 conversation with FAA Administrator M. Huerta concerning drone safety and integration into the NAS.)

See 91.905, List of rules subject to waivers:

91.119   Minimum safe altitudes: General.

In other words, 91.905 allows the FAA to issue a waiver to 91.119, e.g. for airshows. The form for requesting a waiver is here, and the FAA can set more or less whatever conditions they like for granting one.

Would a pilot require/receive a standing waiver for close-proximity flight near authorized structures, i.e. cropdusting on one's own farm? Relatedly, what about landing near a campsite out in bush country?

@KennSebesta Cropdusting is a specific activity with its own rules. And landing is already an exception to 91.119. The real bottom line is 91.13: even with a waiver 'careless and reckless' still applies.

@757toga I would consider locating a campsite 500' away from the landing strip to be a burdensome requirement more often honored in the breach than in the observance. But then again I've never landing on anything other than a several-thousand-foot-long designated runway so I really don't know much about it.

@KennSebesta - If you are landing, it does not matter if the campsite is within 500’. All of the altitude and distance requirements are applicable except when taking off and/or landing. Even at some small paved or grass airstrips, there may be people, vehicles and/or structures less than 500’ below or around the aircraft. A mower mowing the grass at the AER threshold is a good example. ATC at a controlled field may just give you a warning of their presence and still clear you to land.


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What drives the mechanical challenges which make trailing-edge flaps an (almost) ubiquitous solution vs leading-edge flaps?
(Apologies for the mouthful of a title.)
I was thinking about canard aircraft and how trailing edge flaps (TEF) are actually counterproductive for them. Leading edge flaps (LEF) could theoretically counter the problem, by unloading the canard instead of further loading it. Great!
But aside from on massive aircraft, LEF are rarely, if ever, seen. I'm making an assumption that this is because of the complexity of developing LEF, an assumption which gets some traction when comparing the 747's leading edge flaps to the DC-3's trailing edge split flips.
Assuming my assumption is correct, what is it about aerodynamics and structures which makes LEF solutions so much harder/more complex than TEF ones?
Please note: I'm not asking about leading-edge slats, which are sometimes confused with leading-edge flaps.

Suggested simpler title: "What are leading-edge flaps uncommon?"

@BenVoigt I had something similar, but that could stray into why they're not worth it. Since my thought pattern was on canards, which are already uncommon, the answers I'm after is why they're tough to build, not just rare. If they were easy to build in a manner that didn't greatly impact payload/performance, they would presumably be more common even if their value wasn't high.

Leading edge flaps are less efficient than trailing edge flaps. Efficiency here means the ratio of change of lift to change in flap angle.
With every flap deflection, both camber and angle of attack of a surface change. While, for example, a nose-down deflection decreases angle of attack while increasing camber such that both effects work against each other, the changes in angle of attack and camber of a trailing edge flap work in the same direction.
Also, a leading edge flap needs to be connected to the wing at its rear end which is a poor choice for a hinge line: The added forces due to deflection will drive the flap to larger deflections. This means it is unstable, always attempting to run into the stops at maximum deflection. To keep the flap under control will require heavy actuators. At the trailing edge the hinge line is ahead of the added lift forces and this will drive the flap to a neutral position regarding tail surfaces while the interconnection of ailerons will keep both sides equally loaded. Trailing edge flaps are inherently stable.
Leading edge devices are only helpful to increase the maximum angle of attack: While the whole airplane will adjust angle of attack to compensate, the flap will reduce the suction peak at the leading edge so flow separation is delayed. This is particularly helpful when you combine slotted flaps inboard with regular ailerons outboard. Without a suitable leading edge flap ahead of the ailerons, the circulation added by the inboard flaps will make the outer wing stall before the inboard wing reaches its full potential. This leads to rather unpleasant stalling characteristics, something to avoid especially during approach and landing.

Thanks, that's an interesting point about the effect of LEF to increase AoA. There are some things here I don't quite get, though. 1) How much important to give to efficiency? Decreased efficiency is a plus when landing, so this seems like a useful factoid but not a reason why not to make LEF.

I don't follow your final comment that LEF "are only helpful to increase the maximum angle of attack". That seems to go against https://aviation.stackexchange.com/a/25921/20394 (not your answer, but one you edited), which claims that "LEF... direct the flow downwards for more lift". Do you disagree with that answer?

I don't understand your point about how the forces on the LEF "drive the flap to larger deflections". Do you mean that they will increase the deflection along the intended motion of travel, e.g. suction forces which pull Schempp-Hirth airbrakes out of the wings? Or do you mean forces which twist them out of their
intended position and cause positive feedback loops on pitch?

@KennSebesta 1) For landing we have plenty of choices for "inefficient" TE flaps. I clarified the first sentence. 2) The main work of downward deflection is still done by the wing, but TEFs help with that by allowing larger angles of attack. Where is the contradiction? 3) LEFs are akin to balancing that famous broom on a fingertip - they will try to run into their end positions. This is poor behavior. TEFs are much more benign.

Thanks! Regarding 2), the contradiction is that you say "are only helpful to increase the maximum angle of attack" and the other answer says "creates more lift" via increased camber and thus the same AoA (relative to the newly defined chord line).

Regarding 3, I see what you mean. Of course, we've mastered that with SH airbrakes which have a dramatic impact on flight if accidentally deployed, so there might be a way forward but I get your point.

Also, as an aside, sometimes it's not good that the flaps have aerodynamic forces pushing them back toward their stowed position. I had a very scary landing one time when, during the flare, the loads on the flaps back-drove the mechanism, causing the flaps to retract and an instant and complete collapse of lift. Fortunately it was an unusually precise flare vertical distance-wise, and I was landing on soft grass. The upshot is the extremely rapid collapse onto the runway didn't have time to develop into a lot of energy.

@KennSebesta Schempp-Hirth airbrakes are no leading edge devices but speedbrakes. Yes, extending them not into, but perpendicular to the local flow is an improvement, but here I thought of "real" leading edge flaps like those you find on the F-104. And flaps retracting all by their own means the locking mechanism was worn out and needs some attention. Or did you inadvertently touch the flap lever?

@KennSebesta Regarding 2) The angle of attack will very quickly find the right value. No need to worry about it. This "more lift creation" is by way of the airplane adjusting AoA all by itself. Only when you exceed the maximum you have really a reason to be worried.

@KennSebesta: Regarding your scary landing, while I'm neither a pilot nor an aircraft designer, I have a suspicion that if leading edge flaps had overloaded and slammed suddenly to max deflection (or beyond!) in a similar situation, that might well have stalled the wing and also resulted in quite sudden loss of lift. (Also, I'm glad to hear that you survived the incident!)

@Peter, the mechanism on this glider was is designed in such a way that when exerting the considerable force required to go full flaps the "hook" can not fully seat. The answer is to always jiggle the handle to make sure it has properly seated and isn't just binding due to light friction. Re SH airbrakes, I refer to them only as control surfaces which do not retract on their own (and at high speeds can be literally impossible to retract without damaging something).

@KennSebesta JAR 22 demands that airbrakes must be safely used up to speeds of 1.05 v$_\text{D}$. I know people who fly faster than that, but would not recommend doing so. What glider is that?

@Peter, I'm not sure that's exactly correct. CS 22.697 requires "It must be possible to extend the device at any speed up to 1·05 VNE and to retract the device at any speed up to VT, but not less than 1·8 VS1, with a hand force not exceeding 20 daN." https://www.researchgate.net/publication/339271817_Dilemma_resolved_Airbrakes_tamed gives some more in-depth reading on the matter, stating "Once opened, the airbrake will stay fully extended throughout the re- covery from the dive - because in most sailplanes, the pilot is not capable of retracting the airbrakes unless at lower airspeed. "

@KennSebesta Yes, that is correct. I had not looked up the paragraph but only remembered the limit speed. As a designer, I would not only look at the extraction but at the full range of operations. It does make sense to limit retraction to lower speeds, but is unwise in real life.

More than mechanical challenges (if any) it's actually basic aerodynamics that

makes trailing-edge flaps an (almost) ubiquitous solution vs leading-edge flaps.

From this answer:
The main purpose of a leading edge and/or trailing edge device is to change the camber of the airfoil. Without entering in physical/mathematical details, it can be shown that, going from the leading edge toward the trailing edge of an airfoil, the contribution of each piece of its camber to the aerodynamic force is proportional to the following plot/equation (my own plot of the function $\sqrt{\frac{x}{1-x}}$):

There it can be seen that the contribution to the aerodynamic forces of the forward part of the camber is basically negligible while it is the 30-something% most rearward portion of it which almost entirely contributes to the aerodynamic forces. This explains why spoilers, ailerons, rudders, ... whatever surface used to change the aerodynamic forces is placed toward the end of the airfoil and not at its leading edge.
This also tells us that, whatever their name is, any high-lift device placed on the leading edge (like slats or Krüger flaps) does not alter significantly the aerodynamic forces: they are indeed used to improve/extend stall characteristics at high AoA but have almost no impact on the (slope of the) lift or on the pitching moment, as clearly visible in the following plot from this Boeing report:

This is interesting. Why, if the last part of the airfoil is the most important, is it so critical not to trip into laminar flow from the LE, esp. due to bugs/dirt/surface roughness?

I also have the same critique as of Koyovis's answer, which is that I'm asking about leading-edge flaps, not leading-edge slats. Unless I'm mistaken, they are not the same thing and do not work in the same way.

A modern jetliner wing might have laminar flow on no more than 20% of the chord, definitely less at the end of the day when dirt has accumulated. Leading edge flaps (or, more generally, high-Lift devices) would basically give no contribution to lift (see plot in my answer), only to $\alpha_{stall}$

@KennSebesta you are mistaken. Flaps and slats work broadly similar, only the details are different, but for calculations we act as they are the same but "energize" the airflow to keep it from getting unattached.

I agree with @paul23

Mechanical challenges - not so much, they can be solved as many existing aeroplanes demonstrate. Flaps and slates have slightly different functions, as shown in the graph below, from this answer.

Trailing edge devices increase lift at a given Angle of Attack (AoA), which is desirable - but they reduce the critical AoA.
Leading edge devices increase critical (stall) AoA.

Particularly aeroplanes with a high subsonic cruise speed have a large disparity in $C_L$ requirements: low $C_L$ in cruise due to the large airspeed, high required $C_L$ during approach and landing. Landing is the critical phase, most high subsonic jets do require the additional $C_L$ and AoA safety margin that the leading edge provides.
Again, mechanical complexity is not really the driver here, aerospace engineers are usually pretty competent at solving complex problems . The triple slotted trailing edge flap on the Boeing 727 was mechanically pretty complicated...
---Edit---
Please note that the leading edge Krüger Flap functions the same as other leading edge devices, as for instance cited in my go-to book on aeroplane design, page 256:

KRUEGER FLAPS perform in the same way as slats, but they are thinner and more suitable for installation on thin wings. Krueger flaps are often used on the inboard part of wings, in combination with outboard slats, to obtain positive longitudinal stability in the stall.

So it seems that the Krüger flaps are called flaps because they flap outwards when deployed, not because they function identically to trailing edge devices. Also, the above Torenbeek citation identifies another function of the Krüger flap: it creates a nose-up moment, partially compensating for the nose-down moment of the trailing edge flaps.

This is all true, and interesting(!), but my questions specifically asks about leading-edge flaps and not leading-edge slats. Leading edge flaps, unless I'm mistaken about other sources I've read (e.g. https://en.wikipedia.org/wiki/Krueger_flap), perform the same function as trailing edge flaps in enhancing lift at the same airspeed/AoA.

@KennSebesta The linked wiki seems to refer to the similarity in functionality between slats/slots/krügerFlaps: While the aerodynamic effect of Krueger flaps may be similar to that of slats or slots .... they are deployed differently.


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Difference in AMP and MPD between two aircrafts
Can two aircrafts such as the A320 and A330 have the same Maintenance Planning Document but different Aircraft Maintenance Program?

Related: What is the difference between Maintenance planning document(MPD) and Maintenance Manual (AMM)?

Do A320 and A330 have the same MPD? I don't think so


text:
Is there a minimum number of people needed for an aircraft pushback?

This seems a bit wasteful. Is there a legal requirement for a minimum number of people and if so, on what basis does this vary? (airline, airport, aircraft type, etc)

If you don't have "wing/tail walkers" you get accidents. Better to waste some time than damage some aircraft.

BTW, when you ask if there is a "legal" requirement, you need to tell us what country you are talking about. This may be different for the US and Botswana, for example.

Hmm. I think I'd be curious to know even if there was a legal requirement in any country.

@Ron Beyer standard pushback in my base (Helsinki, Finland) is done with one person on the ground, in the tractor to be exact. We haven't had any pushback accidents that I'm aware of. We have an excellent safety culture and very good training for everyone involved. Wing walkers might improve safety, but their absence does not automatically lead to accidents

@Sami The stands at Helsinki are set up such that you aren't pushing back into other aircraft, so the chances of having those types of accidents are much lower. Look at an airport like O'Hare, where there are "alleys" that the aircraft park in, this pretty much requires it. Even my small airport has wing-walkers just for safety reasons.

Regarding your picture, some of the people are probably trainees

It's not just on pushback... this happened, probably because no-one was assigned to keep watch where it mattered. https://www.youtube.com/watch?v=8-UGnbtX67M

Depends on the airline.  At minimum, you need one driving the tractor, and in unconfined areas with nothing to run into, this is sufficient.  In confined areas operators almost always require wing walkers to clear the tips, and someone at the tail if backing into a confined space or to make sure the pushback stays within any boundaries.
So the answer is, anywhere from 1 to 4, depending on the situation and the airline's internal policy.  Insurance requirements will have an influence on this in a lot of cases because the results of screw-ups are usually just insurance damage claims with nobody getting hurt.

One can easily cause damage worth a years salary of a wingwalker by not using one.


text:
Texts on aero-engine structures
I have been scouring the web for resources on aero-engine structures and structural design (preferably civil turbofans) but unfortunately it seems like they are hard to come by. I would like dedicated texts on stuff like engine loads, what the major load-bearing components are, engine-airframe integration, what modelling techniques are used (from simple analytical/empirical models in prelim design all the way to perhaps advanced thermomechanical FEA), what the design considerations are etc.
I figured that most of this info might not be in the public domain because of trade secrets and such, but I was wondering if anyone could still point me in the right direction. I've been trying to piece together my understanding through reading some of the AIAA and ASME papers and even some of the OEM patents but would also like more comprehensive texts like the ones in https://www.ae.msstate.edu/tupas/SA2/Reference.html for general aircraft structures.

Try the NASA Technical Reports Server. You can access the archive of NASA reports though it may take some searching to find what you're looking for.


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Are the PW1100G and PW1400G engine series not certified by the FAA?
I was digging around the FAA type certificate data sheet (TCDS) database and I could not find the entries for the PW1100G and PW1400G engines series. I then went to the EASA TCDS database and both of them were categorised under the PW1100G-JM title. Have these two engine series not been certified by the FAA? Was it because they were for European aircraft OEMs (PW1100G for A320neo, PW1400G for Irkut MC-21)?


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How is the control surface configuration of predator/reaper for redundancy system?
In commercial aircraft like Airbus or Boeing, there are several control surfaces that work in the same function such as for roll control capability, the control surfaces are aileron and spoiler. Also just like (Correct me if i'm wrong) HSTAB trim as backup of elevator. Because of course it is used for redundancy hence there are no single failure lead to catastrophic. But i read that predator and reaper do not have spoiler. So how the configuration of predator/reaper control surface to accommodate "no single failure lead to catastrophic" in term of control surface failure
Thank you

Why would they need redundancy? It's not like it's going to kill hundreds of passengers if it crashes.

@JörgWMittag Okay, but according to CS 23 Special Condition rev 3 link related to RPAS, still, if the RPAS fly into civilian airspace and crash due to erroneous of roll control resulting one or more fatalities, it is still considered Catastrophic. But even below it, which is hazardous, the case is loss of RPAS control capability, which i think the accommodate hazardous type function, redundancy is still required.

Why would this be relevant? Has General Atomics applied for a civilian European Type Certificate? I can't even imagine what a civilian use of a. Reaper would be.

There is redundancy. And, then there are work arounds. The examples that you gave sound more like work arounds. They are systems meant for another purpose that can be utilized as a last resort. For instance, trim tabs are not meant as backups to primary control surfaces. They can be used as such in a pinch. But, that is not their primary function. Saying that, many small aircraft do not have trim tabs on every control surface. Nor, do they have spoilers. Safe flight is still accomplished. Military drones are not large aircraft compared to Airbus and Boeing aircraft.

Plus, reread section 3 in the document you attached. https://www.easa.europa.eu/sites/default/files/dfu/SC-RPAS.1309-03_Iss01_FOR%20CONSULTATION.pdf  This section alone makes the rest of the document only applicable to the advanced technology used to make drones possible. It does not apply to the more tried and true technology common to basic aircraft. In other words, drones have to follow the certification process of your basic aircraft for normal flight as if they are manned. Plus, they have to adhere to further requirements only for those components needed specifically for unmanned flight.

You may note that redundancy is also provided by several control surfaces moved by different hydraulic circuit (e.g. multiple spoilers, each pair being connected to different hydraulic circuit)

I don't think “no single failure lead to catastrophic” is ever actually true. E.g. main rotor gearbox in a helicopter is a single point of failure, as are many structural elements. The real requirement is that probability of a critical failure must be qualified to less than once in 10⁹ hours and that can probably be done with independent actuators on the single surface. It is what is done for the rudder and there are aircraft that have stabilatosr, e.g. Lockheed L-1011 TriStar, so they don't have any second surface for pitch control either.


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What exactly does the MB-326's 'relight' button do?
The following picture shows the engine throttle of an Aermacchi MB-326, which is powered by the Rolls-Royce Viper turbojet:
(source - all photos © Drive&Fly Model Club)

I wonder what exactly the RELIGHT push-button is for. It's clearly not the 'reheat' control (another word for afterburner, which is not fitted on the Viper, apart from some amateur variants...)
I would guess it controls the engine ignition for use in case of flame-out, but then I noticed this other photo, showing what appears to be the starter/ignition controls (located in front of the throttle):

so my question is: what exactly does the relight button do, as opposed to the starter/ignition controls?

Early turbojets such as the Viper had a tendency to flame out. The relight button was used to restart the engine in the air.
Source


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What is the climb-out angle of a twinjet upon one engine failure scenario?
Take a typical 737-800 or A320 at MTOW how much difference in take off angle does a 1 engine out failure scenario make, compared with a normal 6-8 degree take off angle with full engines.

Are you asking about a climb angle or gradient, or are you asking about a pitch angle?


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Why do we still need ammeters?
I just received my private pilot's certificate 2 weeks ago, yay! Earlier today, a fellow at work who has 700ish hours walked in and asked my office-mate-electrical-engineer "why do we still need ammeters, cars did away with them years ago, voltmeters seem mostly enough." He happens to be nearly done building his RV-14a. Office mate and I were at a loss for making a compelling argument for why aircraft still use ammeters. Is it just for redundancy's sake?

Welcome to aviation.SE, and congratulations on becoming a pilot!

This is a good question.  These answers are helpful, but I would add one note.  Not all ammeters indicate the same way.  Some show charge and discharge of the battery, which is very helpful, because it shows that the alternator is keeping up with the load.  Another variety shows total alternator output, which helps you understand the load and what the power hungry accessories are, but it does not make it obvious that the battery is charging.  It is worth spending the time to understand which kind you are looking at and what it is telling you.

FWIW, cars didn't really get rid of ammeters - people just got used to the voltages of lead-acid batteries and can guesstimate current flow from reading the voltage. Now that we are moving to electrics cars basically have ammeters again but instead of displaying amps the on-board computer would calculate watt-hours or kWH or equivalent miles per gallon.

I read this as Altimeters in the HNQ - that would be a brave question!

Congrats on earning your wings!
Because ammeters measure the flow of current (positive or negative) they can immediately show when the alternator is no longer providing power and the battery is discharging instead. In a voltmeter, like most modern automobiles have, you would have to wait to see the charge level on the battery drop, and by that point, several minutes (or longer) may have passed. In an aircraft, where it may be important to quickly shutdown electrical devices and conserve battery power, every moment counts and the ability to see flow shift from positive to negative is very helpful.

I don't think you'd have to wait.  If the alternator craps out, the system voltage will immediately drop from 14 volts or so to 12-ish, then slowly decline from there.  However, to your point, this change is more subtle and much more easily overlooked than the movement of an ammeter from 0 or slightly positive to negative.

@JohnK: That is correct. However, remember that modern batteries are not made to gradually decline. They are made to remain as close to 12 as possible for most of the charge, and then drop steeply just before they go dead. Otherwise, you would have to shed your device’s as the battery drained to keep them from dimming and blinking in and out.

Agreed but you still get that drop from 14-15 to 12-13 as soon as the alternator stops.  Most ppl wouldn't catch it however, so your point is still valid.

All in agreement.

Plus, it let’s you see exactly how much you are drawing from the battery if you want to shut off part of the equipment and leave part of it running. You don’t really need to do that kind of fine tuning in most automobiles.

Err... Most modern automobiles don't have voltmeters or ammeters.  (Unless you hook up an OBDII reader.)  An idiot light is all you get.

@jamesqf: That... is sad, but very true.

@jamesqf : indeed, but such problems in a car are much less dangerous than in a plane. Even in the worst case you can just pull over.

@vsz: Well, no.  The worst case is that you're stuck somewhere (like on the other side of the Black Rock Desert, maybe) with a truck that won't start 'cause the idiot light didn't come on when the alternator failed.

@jamesqf I'd rather be in the truck on the side of the road than in the plane a few thousand feet above with the same problem.

To avoid any confusion, I just want to point out that even cars with only an "idiot light" will still technically have a digital voltmeter working behind the scenes which provides the data that's used trip the light when something goes wrong.

@JohnK Alternator failing first is also the better scenario.  Worse is your battery failing first - then you're flying along with 14V until bam - alternator goes and now you're dead with no warning.  Hopefully nowhere near IMC...  An ammeter would have warned that the battery was no good.

@vsz, Re, "You can just pull over." Not always. I was maybe 60 miles from home when it finally sank in that the alternator in my car had crapped out. I figured, the closer I can get to home before I need a tow, the better; so I pressed on. I was three miles from home when it died, on a busy, elevated, up-hill exit ramp. And that's when I realized, "Oh! If the battery is dead... then the 4-way flashers... won't work." Oops! Still, probably better than than an engine out with nothing below but forest for as far as you can glide.

@Alexandre Aubrey: I'd rather not be in either situation, myself.  But if you have an ammeter, you can tell if it's happened, and with luck before you wind up in a bat place.  Indeed, I've had just this situation happen in an airplane.  Alternator quit due to a broken belt just before crossing the Sierra (westbound, so I would have been in a lot of controlled airspace with no radio).  Instead, I just landed at Truckee and had a new belt installed.

@AaronHolmes: In many older vehicles, the car's "alternator idiot light" bulb served to limit current in the alternator field coil, since when the alternator wasn't turning the field coil would pass maximum current while doing zero useful work.  An interesting quirk of this was that if the bulb itself failed, the alternator would cease to generate power but the bulb itself wouldn't light (for obvious reasons!).  The only way the driver could notice the failure could be by the light's failure to illuminate when the engine wan't running.

Because it displays how much current, and thus how much power, your electrical system is consuming.  This provides a quick reference for diagnosing problems.   Negative values can indicate  a malfunctioning alternator and large power draws can indicate a short circuit somewhere in the system.

Ammeters on cars and airplanes only show net current surplus or deficit to or from the battery in amps, not the total current load imposed by electrical services.  When the battery is fully charged and the alternator working normally, the ammeter will show 0 or a very slightly positive current, only what is needed to keep the battery charged, regardless of the service load.

Yes, many aircraft have ammeters like cars that show positive or negative current flow. I have also flown aircraft with ammeters that only show the actual load on the generator or alternator.

@JohnK strange, because I’ve seen plenty of ammeter which will display the current load.

If the alternator is off.  If it's running it shows the charge rate or discharge rate of the battery not the total load. If your ammeter is showing say .5 amp charge rate with a fully charged battery, and you pile on the load, the alternator's current output will go up to compensate to maintain the system voltage, but the ammeter will still only show .5 amp if the battery is charged.  The only time it shows substantial current is when the battery is depleted and is being charged, like after starting.

IIRC, Piper cherokees had an ammeter that showed positive / negative values as John K indicated.  Business jets typically have ammeters that show the output of the generators as Mike and Carlo indicated.  I don't know if one type is better than the other

Yes that's because the ammeter is wired across the battery current source, not across the alternator current source.  It only shows current flowing in or out of the battery.  The only time it shows the total load is when the battery is the only current source.

The more unpleasant a failure of some system is, the more tools you get to prevent, diagnose and mitigate the failure.
An ordinary passenger car can stop in a middle of nowhere because the alternator failed and the battery depleted. Quite unpleasant, but rarely fatal - and modern cars tend to be reliable enough anyway.
An offroad rig with winches, extra lights, radios, compressors, etc... usually has not only an amperemeter, but a full-blown battery controller with voltmeter, amperemeter, ampere-hour metter (integrating amperemeter) and so on. If you are out of electricity, you are in a quite worse position than in a passenger car. And you have quite a deal of extra failure modes.
Same for marines. They adopted these controllers even before the offroad cars.
In aircraft, "no electricity" situation is even worse, but you have the extra limitation of not overloading the pilot with extra information that they rarely need. That's why volt + ampere is okay-ish.

The reason ammeters were dropped in cars is they were (and are as used in airplanes) wired to the battery and only show the current flow into or out of the battery.  The battery's nominal voltage is 12, but the system operating voltage is about 14, and the voltage regulator varies the alternator field to keep roughly 14 volts in the system so that the battery is always getting some small amount of charging current. The current flowing from the alternator will be whatever is needed to sustain 14 volts under the demand of electrical services plus charging, or keeping the battery charged.
With the alternator running and a heavy electrical load on the main bus, the alternator's output may be, say 10 amps, with most of that going to the lights and heaters and radios etc, with a small surplus going to the battery if it's fully charged, whatever it needs to stay charged.  The ammeter is showing what is going to the battery and it should normally be 0 amps or a slight positive surplus once the battery is recharged following a start (because the 12 volt battery is connected to wires with 14 volts in them, so there will be some trickle flow through the battery even when fully charged).
In cars it was decided that a voltmeter was sufficient because if the alternator is running it will show roughly 14 v in the system, and if the alternator is failed it will show somewhere around 12, at some point declining further from there until it's down below 10 volts or so and everything stops working.  So the voltmeter is sufficient to tell you the alternator is running and sustaining system voltage, or not.
Having the ammeter there as well provides a more complete picture because it's showing current to and from the battery as well as system voltage.  For example, when you start, the engine is running with the alternator is putting out current to charge the battery that was depleted somewhat by the start.  The voltmeter will just show 14 volts, but the ammeter will show the heavy charging current going to the battery, which can be good information to have if you want to be aware of the battery condition.  You will normally see the temporary heavy charging current for perhaps a minute or two until the battery is brought up to charge.  Charging current the goes much longer than this could be a sign of problems in the battery, like high internal resistance.
Similarly, when you turn on heavy load services with the alternator running, the ammeter reveals the temporary current drop at the battery resulting from the voltage drop from the service taking "juice" off the bus, and there is a slight delay as the voltage regulator reacts and raises the alternator field to put out more current.  The ammeter will show a short discharge spike, then recover. That's the voltage regulator in action, and it can be good information to know.  The voltmeter will show a voltage drop/recovery as well but it's more subtle.
Bottom line is, once you learn to interpret them, having a voltmeter and ammeter together provides a more complete picture of what your electrical system is doing and its overall health.  And pilots are expected to be more concerned with that sort of thing than drivers, so there you go.
With cars, where you drive it until it won't, not such a big deal unless you're a hardcore enthusiast, so why not save a couple bucks by deleting one of the meters. And overall, if you have to give up one of those meters, you will generally prefer to keep the voltmeter because it's information is more important of the two in the general scheme of things.

I believe "wired across the battery" is a bit misleading. A more correct term would be "wired in series with the battery." If the ammeter was wired across, or in parallel with the battery it would present a dead short to the battery and the charging system.

It wasn't really meant to be taken literally, but yeah. Edited.

Some history from the automotive side, and a bit of information from the electrical engineering side.
To measure amps, you need to have the meter in series with the load.  Earlier systems had the amp meter directly in line, so all the power of the vehicle ran through the ammeter.  This could be dangerous if something went wrong, as you had 30+ amps running through the gauge.  Later years, they used a shunt.  A wire between the battery and a junction.  The ammeter measured across the shunt and scaled the reading based on resistance.  This is safer, as you can measure 60 amps, with less than 5 amps running through the gauge.
Measuring voltage is MUCH easier.  Voltage is wired in parallel.  Just wire one side to the key on power, and the other side to ground.  On a car where the gauges are controlled by the ECU, the ECU is already getting a voltage signal from the battery that it can measure.  As some have said, at this point, it is usually a light rather than a gauge.
The change from ammeters to voltmeters came around the time they switch from a generator and voltage regulator to an alternator with an internal regulator.  I don't know if this is just a coincidence.  The alternators tend to be more reliable, and provide more power at low RPM (idle).
So in terms of automotive, I think the main reason for the change is simplicity and cost.

practically, an ammeter is but a voltmeter measuring the voltage drop across the small (ideally: zero) resistance of a part of the conductor carrying the current we want to measure.

An ammeter can be Hall-sensor based, they don't rely on voltage drop. A modern shunt-based ammeter can work as low as 50mV drop for 500A - less than the drop in cables.

... cars did away with them years ago ...

I'm working in car electronics sector and I know that your assumption is wrong:
Modern (combustion motor) cars use a so called "battery management device", a computer which measures the voltage and the current of the battery; in some cases it even measures multiple voltages inside the battery and multiple temperatures of the battery.
The truth is that the display has been removed from the cars because the current is monitored by the computer. If there are problems with your car battery, you will typically see some warning message in the car's dashboard.
Why is this not done in airplanes?
My suspicion:
The safety regulations for avionics are much stricter than for car electronics (*). Therefore, building electronics for airplanes is much more difficult and more expensive than electronics for airplanes.
So if it can be avoided to use some electronics in a sports airplane, it is avoided.

... voltmeters seem mostly enough.

Especially if the battery is old (and bad), a voltmeter does not give you any information:
A voltage of 12.5 volts may indicate a full battery being discharged or an empty battery being charged.

(*) Just one example: In passenger airoplanes the infotainment devices at the passenger seats are already classified as "safety critical" (DO-178B level "D").

Yes a voltmeter indicates state of charge with some provisos. It shows issues like insufficient charge and excessive load by implication. Simply put, an analogue meter needs offset and expanded scale to show much detail, and responds to charge anomalies more slowly. It needs to be accurate. An ammeter in the battery line indicates charging or discharging more assertively, and shows the important states clearly. Not charging, discharging or well charged (assuming the charge voltage is correct for the battery). With the engine/charger off it could show load current.
An explanation of the electrical side is to consider there are three connection points, the charger, the battery and the load. These could all be wired to meet at a common point on the active side, so they are all in parallel. Additional chargers like solar panels and portable generators, and batteries would each connect separately to that common point. In some cases blocking diodes may be needed. A single ammeter can be connected in series with the battery, the charger or the load, indicating:

Battery charge or discharge, with the direction of current in or out of the battery. The current shown does not indicate the charger or the load current directly, but the battery current. This is the traditional usage, because it indicates a charger or a load issue. It does not need much accuracy to be useful, so long as it reads zero properly.
Charger current, which shows the sum of the battery and the load current. It shows if the charger current is sufficient. Note the charger is ideally a current source with a voltage limited to the maximum allowed battery voltage. This means nominally constant current once the battery voltage is exceeded by some small margin, until the maximum voltage is reached, so the need for this meter is not great. In fact the voltmeter indicates what a charger is doing. The battery holds the charger down to the battery voltage.
Load current, which is just the load itself, part of which could be from the charger or the battery. It shows the load current is approaching limits. Normally the load would be within the charger limits for a vehicle, aircraft or boat, with the battery supply brief surges in current that exceed the charger capability.

The starter is usually wired separately so the current does not pass through the ammeter. The other side of the supply may be ground return (using a metal frame as in cars) or use go and return wires, with a single ground point. All the load, charger, battery, starter and connections would be insulated from ground. The regulations or codes of practice for particular systems will spell out how to do it, reducing the need to be a design expert.
It is also possible to use current shunts, and a switched ammeter (really a voltmeter calibrated in amps). This can help reduce the amount of heavy current wiring to solve layout problems. This would give a more complete diagnosis of where a problem lies, along with a system voltmeter. No one would bother with a car, but for a boat? The battery current ammeter shows current in charge or discharge direction, and could be simplified to a few lights, charge current, discharge current, very little current. In the past the ammeter did not need to be accurate, and acted more like three lights. They were cheap and rugged. A useful analogue voltmeter was expensive and delicate. An accurate, expanded and offset scale type would be unusual at least before the days of zener diodes. Though I do not doubt it was possible with an appropriate magnetic circuit it would have been unattractive in a car, boat or aircraft. Nowadays a digital voltmeter overcomes this easily. Personally, I would prefer the center zero charge ammeter or electronic equivalent, with decent digital battery voltage meter. Visible in all lighting conditions of course. There was little choice in the past, a battery ammeter with center zero was the choice. Nowadays a voltmeter can be used. Both methods need interpretation, and probably it depends on who is using it.

Voltmeters tell you when your battery is flat.
Ammeters tell you when your battery will soon be flat.
Your battery is being charged by an alternator, only an ammeter will tell you if the alternator is working. If you don't have an ammeter then you might wait several hours before realising you've got a problem


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90 minute youth activity, what to do?
This is a bit of an open ended question.
Every year or so, one of the youth groups associated with local churches will want me to come and do some evening youth activity with them about aviation/engineering. The easy solution is to just lecture aviation things at them, but I find that's not very rewarding for me or them. The recent request is to spend some time, 90 minutes-ish, on a weekday evening, with young men, age 15-18, 8+/-4 youth. It's too many (and too cold) right now, to do much out at the local airport I think. We could go look at the plane I fly, but.. meh. There's only so much I can do with paper airplanes and aerodynamics.
I'm soliciting a format suggestion for what to do that would be entertaining/engaging/educational. I could possibly break whatever I did up into 2 different evenings if it were a longer format.
(feel free to slap some better tags if there are any on this)

I’d love to look at airplanes and airports and have somebody explain how it all works to me. I also think you underestimate the appeal of going somewhere where the normal public can’t go.

Do you have experience building model airplanes other than paper airplanes? I mean, the balsa and tissue variety. A classic AMA activity is to build the delta dart: https://outerzone.co.uk/plan_details.asp?ID=7931 . It's fairly easy to build and you can teach them about trimming.

I sometimes do a charity day taking disabled kids (& their families) up, for about fifteen minutes, in my PA-28.  It's surprising how amazed and awestruck they are by it.

@slebetman I think that would take way longer than 90 minutes but is on the right track. Perhaps simpler that could be built much faster so they could spend most of the time launching them and trying to adjust the CG and trim, rather than building them. For example, just stick fuses with monolithic card stock plank wings. Maybe bendy wire like the way pipecleaners are that they could use to re-adjust the trim. If too simple then maybe add a rubber band propeller. I think the time spent adding a propeller would be more interesting than cutting/gluing balsa sticks and covering wings.

"We could go look at the plane I fly, but.. meh" - That sounds absolutely fan-bloody-tastic. Do that.

@DKNguyen The Delta Dart could be built in about 5 minutes especially if you bring pre-cut kits. It's just a bunch of flat triangles. IMHO the only real complicated part is making the dihedral. If you want to avoid that step another easy to build model is the Squirrel: https://www.rubber-power.com/make-it.htm - no dihedral, all straight lines and 90 degree angles. The Squirrel is probably easier to build since you can build it directly onto tissue avoiding a separate covering step. My only bias for Delta Dart is because I think it flies better but both are good beginner builds

I would absolute love to look at your plane. And can they take turns sitting in the plane? Can they (or you?) move control surfaces or even turn on the engine (safely without flying away)?

the "8-18 y/o young man" (why only boys? take the girls, too!) i was, plus all of my friends at that age would have LOVED a tour around an airplane!
one of my most cherished childhood memories was, when at age 8, i was allowed into the cockpit by the flight attendant and the pilot let me turn the auto pilot's heading knob. imagine doing that as an 8 y/o and the ginormous 737 you're on starts doing something because of it! also, what is being discussed in the comments: absolutely build small freeflying foam airplanes together :)

oh, and since you seem to be in the US, you might want to get in touch with flitetest.com ... the owner is both a real-airplane pilot as well as a practicing Christian, which seems to have some overlap with your own life :)

Play Flight Simulator on a PC or Console
Youth love video games. They get excited when there's competition. Kids (and adults) are fascinated by large transport aircraft, often because everybody's got personal experience.
If I were to do it, I'd get a gaming PC/Xbox, a basic control setup (they have Boeing and Airbus setups for not much money), and a TV or projector. Microsoft Flight Simulator has a bunch of competitive scenarios, for example, landings where it gives you a score.
Start off by teaching them how to play, and this is where the education comes in. For example, controlling the flight path leads to a discussion of AoA and control surfaces. Controlling speed, you show them the engine instruments, and you discuss how an engine works. Teaching them how to read the FD leads to a discussion of avionics. Maybe you have a few slides about this.
Letting the students play is fun and competitive and you can insert lessons. For example, kid pulled up too hard and stalled, now you briefly talk about that. Player lands great, so turn off ILS and teach them how to read the PAPIs.
Even though it's just a simulation and a video game, it feels more authentic than paper airplanes.

I run FlightGear on my laptop, which definitely not a gaming PC (8 GB RAM, Intel Core i3, integrated graphics), with no issues, although I only fly GA planes, not airliners, so airliners might need a more powerful computer.

Won't this have the issue of being able to accommodate only one of the children at a time, while the rest can only look? And if there are many of them, then they will individually have a short time at the controls.

@vsz Sure, but with the worst-case 12 participants x 5 minutes = 1 hour, less than their allotted time. And also watching and commentating on others playing video games is popular entertainment: see Twitch.

I had a similar idea, preferably if you own a recent X-Plane: Maybe make a competition who is fastest on a circle around the airport (i.e. takeoff, circle and land on the same runway again). You could try the Cessna, the F-4 (Phantom) and the F-35 just to demonstrate how different those are. Finally you could also use a helicopter. And for the winner discuss why he/she was the fastest. You could also do a boring lecture on real-life regulations ;-)

This was exactly my idea. Anyone can play MSFS, but barely anyone with commentary from the real pilot. There is an enormous amount of things you got used to, but a lot of non-aviators have no idea about. Like, conditions and consequences of icing. VFR into IMC. The ability to land propeller plane with no engine. The ability to land a helicopter with no engine. I am no pilot, but those were really interesting when I learned about them. I bet you can list a number of them. And then do a competition of a simple landing. Each attempt takes around 2-3 minutes, so a lot of people will be able totry

I'm both a pilot and an Eagle Scout, so I'd love to help your 90 minutes be spectacular.
I would break this into three sections with hands-on demos, which will keep things moving, and expand their minds a bit.

Bernoulli effect that creates lift above a wing. And stalling.
The balancing act between speed and horizontal stabilizer down-force.
Basic Radio: Who you are, where you are, and what you want.

Your best bet is to explain the science behind it, then hands on demo.
Come prepared with some basic props for each section, have them work together maybe in groups of two or three.

Regarding the basic radio, it might also be interesting to introduce one of the live ATC sites. Tune in and listen to the radio at a major airport.

Re 1. a) I do not like the use of the word effect here, it is much better to talk about the Bernoulli's equation or Bernoulli's principle. It is not really an effect, it is a conservation law. b) One has to be really careful as way too many demonstrations are principally wrong and show flows like jets and compare what happens inside and outside of the jet. The B. principle only applies at distinct points along a single streamline. It does not say anything about what happens on different streamlines. For stalling, the Bernoulli is already problematic as turbulent mixing typically happens.

When listing to the radio, you can simultaneously watch flight radar to see movement of the planes mid-air or on the ground.

For what it's worth, no explanation of lift involving Bernoulli's principle has ever made sense to me (and I'm a pretty smart cookie who knows the fundamentals of aerodynamics pretty decently). You can start with the premise that the air above the wing is moving faster (relative to the wing), along with some other premises, and eventually conclude that there must be lift, but that leaves a critical question unanswered: why does the air above the wing move faster? I've never heard a correct explanation of that.

@TannerSwett It's worse than that. Bernoulli's principle is based on conservation of energy: if there's no other source of energy, then any increase in kinetic energy (velocity) must come from a decrease of internal energy (pressure). But how can one possibly take the energy of the air to be constant? The whole point of the wing is to exert a force on the air. I find the laminar explanation more plausible: the air sticks to the wing, so since the end of the wing is angled down, the air leaves the wing with a downward velocity.

@TannerSwett You need a force to accelerate something with mass (F=ma); in case of the air on the upper surface of the wing, this force comes from the low pressure region, which accelerates the air ahead of it (and subsequently decelerates the air behind it during the pressure recovery phase).

@AdityaSharma That's all true, but what I'm looking for is the missing part of this explanation: "[Something], and so the air above the wing moves faster than the air below the wing; given Bernoulli's principle, and the fact that the air far ahead of the wing is all at essentially the same speed and pressure and viscosity is not significant here, we can conclude that the pressure above the wing is lower than the pressure below the wing." If "Something" here is the existence of a low pressure region above the wing, then the explanation is circular and thus doesn't explain anything.

@TannerSwett Exactly. Using Bernoulli's principle, we know that an increase in dynamic pressure must be accompanied by a decrease in static pressure. However, this does not imply that the increase in dynamic pressure is what's causing the static pressure reduction (it is the other way around!). So what is causing the static pressure reduction in the first place? The inertia of the air particles; see this answer for more info.

The Bernoulli explanation ("Equal Transit Time") is — on its own — wrong. The Newton's Third Law of Motion explanation is — on its own — wrong. However... if you have to pick one of the two in a "Lies To Children" kind of explanation, Newton's is a whole heck of a lot better, because it makes sense. (Continued)

(Continuted) I would tell the kids: "Very simplified, the wing knocks the air down, and as such it surfs on the air. Note though, there is more going on, and even people with university education debate on how to best explain how a wing works, because this is — at the fundamental level — extremely hard to wrap one's head around. So be aware: this is very simplified explanation." And here is an absolutely wonderful video to illustrate the gist of things: https://www.youtube.com/watch?v=0HIddtgGzDE

Yes! I was about to say what Rivet said but Rivet said it both first, and better!

Why not paper airplanes?
A contest: "Who can make their plane fly the farthest?"
Then, "Why?" Design, construction...Analyze the 'What made a difference'
The Great International Paper Airplane Book might serve as a starting point.

I’m not opposed to doing something with paper airplanes. Just trying to figure out a good way to interweave it with some actual aviator stuff.

authors: Vladimir F Героям слава


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How is “mountain obscuration” determined?
Living in southeast Washington, ForeFlight often paints large swaths of mountainous terrain to the east as IFR “mountain obscuration”. I live at the foot hills, and often as I look that way, I see only a few spotty clouds here and there. I find this class of IFR very vague/confusing.
Can anyone shed any light on what rubric they use to declare this? Is it simply saying “hey VFR pilots, you shouldn’t go in a cloud in the first place, but the clouds in this region might have solid cores because the highest elevation in this quartile is at or above the broken cloud floor”? Or is it more specific than that?
I’m not looking to bend the rules or push the envelope here. My goal is to be old rather than bold. It’s more that the vagueness of the declaration bothers me; I want to understand at a more fundamental level what the envelope of weather is that fits inside “mountain obscuration” really is.


text:
Make a 3 engine turboprop electric or steam hybrid aircraft?
Was thinking about this, have a 2 engine regular turboprop with a third electric engine for extra performance (imagine a DC3 like the Conroy Tri-Turbo-Three). The 3rd electric engine would be run from the turboprops' alternator/generator and solar, with minimal batteries for emergency use.
An alternate version has the 3rd engine run by steam, the turbine engines produce the exhaust heat to power it, and cold upper air temps will cool the steam quickly.
In either case the 3rd engine does not have to be as powerful, it's just there for performance/emergency.  Does either sound feasible?

Weight weight weight.

upper air is low density air. Cooling your steam is everything but easy. But if the flight Mach number is high enough, those coolers could help to produce thrust.

The thermal efficiency of steam power is not great.  And if you're going to power an electric motor from a turboshaft powered generator, you have losses from the conversion and you might as well run the prop directly from the turboshaft.  Then when you get to batteries, the energy density is only about 1/3rd of fuel, in terms of power per lb of overall weight. In the  end, it's all too heavy.

@user3528438: And thermodynamics.  As in if you use the hot exhaust from the turbines to produce steam, they are not producing thrust.  Also, except for special cases (like surveillance aircraft that need a long loiter time) you are running the engines at something like 80% of full power at cruise.  Hybrid cars work because cruise is 20% or less of full throttle, so there's lots of room to downsize the engine.

@JohnK 1/3rd? I remember hearing something like 1/50

It's higher when you consider batteries alone, but you factor in the weight saving of an electric motor, it improves to around 1:3 or 1:4.  For example, you can buy a number of electric paramotors and their endurance is roughly 1/3 to 1/4 that of gas, overall pound for pound.


text:
I would like to insure my foreign registered aircraft in the USA
I am looking to fly my UK registered aircraft in the USA this summer.  At present UK insurance companies will not insure aircraft flown in the USA and US insurance companies will not insure foreign aircraft.  I see 2 possible solutions:

Find a US company that will insure a foreign plane
Temporarily register my plane in the USA.

Does anybody know if either of these is possible?
Thanks

I don't know if this question will be deemed as on topic for this site; if it is not (it seems to be about insurance), you can ask here and get an answer from people well acquainted with private flying on both sides of the pond.

You could do either or neither.
I assume you are flying a small, piston-engined aircraft. Any insurance company that insures turbine powered aircraft would have provisions for this. I don’t specialize in aircraft insurance. Having been in insurance for 20 years, I know that anything and everything can be insured for the right price. If your insurance company won’t do it, there are others that will. You just might not be willing to pay the price the insurance company requires.
The question you might ask yourself is how do people who ferry aircraft across the pond insure the aircraft? If you are planning a cross-Atlantic flight, you might want to contact an experienced, professional company to help you plan it. You have to think of more than insurance. Communication, water safety/survival and egress, auxiliary fuel planning, weather, sovereign airspace entry, enroute landing airports, customs, and other items need to be considered.
An alternative is to rent an airplane or airplanes here in the US. I have run into a few of Brits, Germans, Aussies and New Zealanders who have done that. They made it seem surprisingly easy and something they did on a regular basis. Some annually. I have even checked into doing it in South Africa. It would have been simple with the renting organization even arranging for validation of my US pilot certificate for flight use in South Africa.


text:
How many sections does a PASSENGER AIRCRAFT belly has? What cargo is allowed in those sections?
How many sections does a belly of a PASSENGER aircraft has?
Like if we take 3 cases :

I am travelling with a check in bag
A person who is not travelling himself but sending some items using cargo booking
A furniture company wants to send 4 chairs to a client

PLEASE TELL me if cargo of all these three cases above can go in the PASSENGER aircraft.
And will there be different sections in the belly for all these 3 type of cargos ?

2 and 3 are the same.

You DON'T NEED to randomly yell DURING your post.

Typically a passenger plane will have 2-3 compartments for cargo. Wide body plane usually has 1 bulk cargo, 1 forward cargo and 1 aft cargo compartment. A narrow body usually contains only bulk cargo compartment as it is too small for standard pallet load.
As to what can be loaded in what location is totally depend on discretion of the airline. They have to consider matter of load and balance of your cargo as well as other cargo. There is no guarantee that where your bag will end up no matter you travel with it or not. The difference is if you are not travel with it your cargo need more inspection to prevent sabotage.

But if I want to send a parcel somewhere and i did my booking with lets say Brussels Airlines, what will happen now? Will my cargo go in any passenger aircraft or in a cargo aircraft?

If you send a parcel separately from booking it can go either way. It may even go by truck. If you are flying and you bring something your luggages go with you.